1. Field of Invention
This invention relates to interferometric systems, and more particularly to interferometric projection systems for quasi-coherent radiation.
2. Related Art
Interference lithography (IL) is a maskless, lensless interferometeric projection technique capable of patterning the smallest possible features at a given exposure wavelength. In IL systems, two mutually coherent (or quasi-coherent) beams intersect at a photosensitve surface (e.g., a resist-coated substrate), creating a line and space pattern with a period as small as xc2xd the wavelength of the incident light. IL has found commercial applications where generation of repetitive patterns is desired (e.g., the manufacture of gratings for diffractive optics, and field emitter arrays for electronic circuitry, and the evaluation and development of photoresist materials). Applications that require more complex pattern generation, such as the patterning of magnetic recording media, are in the research and development stages.
As state of the art photolithography has come to rely on excimer laser sources at wavelengths xcex=248, 193, and 157 nm, the implementation of interference lithography has become more complicated. These lasers produce intense ultraviolet radiation, but have poor lateral spatial coherence, typically on the order of tens of micrometers, and also have significant beam pointing instabilities. Light sources having limited spatial coherence, such as excimer lasers, are referred to as quasi-coherent sources.
In conventional IL systems, which include a beam splitter and two mirrors, the useable field size over which an interference pattern is generated is limited to a size equal to the lateral spatial coherence length of the laser. As stated above, the lateral spatial coherence length of quasi coherent sources such as excimer lasers is only on the order of tens of micrometers; accordingly, the size of the pattern generated by such systems is impracticably small for many applications. Also, such conventional IL systems are particularly sensitive to the beam pointing instabilities inherent in excimer lasers. One such conventional IL system is described in the publication, xe2x80x9cDeep-ultraviolet Interferometric Lithography as a Tool for Assessment of Chemically Amplified Photoresist Performance,xe2x80x9d (Journal of Vacuum Science and Technology, B 16(5), November/December 1988), by W. Hinsberg, et al.
To overcome these limitations, interferometric projection systems based on diffractive optics have been demonstrated. An example of an interferometric projection system based on diffractive optics is disclosed in the publication, xe2x80x9cLarge-area Achromatic Interferometric Lithography for 100 nm Period Gratings and Gridsxe2x80x9d (Journal of Vacuum Technology, B 14(6), November/December 1996), by T. A. Savas, et al. Such diffractive systems include a diffraction grating that splits a laser beam into two beams. Each of the two beams so generated is diffracted by another diffraction grating to cause the beams to intersect to form an interference pattern. The period of the pattern generated is half the period of the diffraction gratings used to cause the interference.
In diffractive systems, the field over which the interference pattern is generated is not limited by lateral coherence length, however interferometric projection systems based on diffractive optics have limitations. For example, the quality of the interference pattern produced by such a grating is limited by the quality of the diffraction gratings that cause the beams to intersect and the degree to which the diffraction gratings are matched. That is, the quality of the pattern generated is affected by the absolute accuracy of the spacing of the gratings elements that comprise the gratings, and the degree to which the spacing of the grating elements of one grating match the spacing of the other grating.
In addition to component-level limitations and constraints, such diffractive interference systems have system-level limitations. For example, the interference pattern frequency generated by a diffractive system is fixed by the grating period of the diffractive optical elements employed in the system. To change the interference pattern frequency, diffractive elements having an appropriate frequency and quality must be fabricated and employed.
Unlike conventional interferometric projection systems, the size (i.e., area) of the projected interference patterns generated by interferometric systems according to the present invention are not limited by the lateral coherence length of the light source, and because interference projection systems according to the present invention are non-diffractive, the patterns generated are not limited by the precision and size of a grating structure. Rather, the fundamental limit on the size of the projected pattern is based on the temporal coherence length of the source and/or the beam size projected onto the photosensitive surface. Additionally, compared to diffractive interferometric projection systems, exemplary embodiments of the present design provide an ability to change the period of the exposed pattern by simply changing the position of elements of the system, rather than by the fabrication of new gratings as required in diffractive systems.
A first aspect of the invention is an interference projection system having an input beam, comprising a non-diffractive first module which reflects and refracts the input beam, and splits the input beam into a first beam and a second beam, and a non-diffractive second module which causes the first beam and the second beam to interfere at a surface, the first and second modules combining to have the first beam and the second beam reach the surface with the same orientation. Optionally, at the first module output, the first beam and the second beam propagate in substantially parallel directions, and at the first module output the first beam and the second beam have traversed substantially the same optical path length. In one embodiment of the first aspect of the invention, at the first module output, the first beam and the second beam have the same orientation. In a second embodiment, the first module TE polarizes both the first beam and the second beam. In a third embodiment, the second module includes a first mirror to reflect the first beam, and a second mirror to reflect the second beam. In a fourth embodiment, the second module includes a first pair of mirrors to reflect the first beam, and a second pair of mirrors to reflect the second beam.
A second aspect of the invention is an interference projection system having an input beam, comprising a first etalon having a first etalon front surface and a first etalon back surface, the first etalon back surface being separated from the first etalon front surface, the first etalon being oriented relative to the input beam to split the beam into a first beam and a second beam, a second etalon having a second etalon front surface and second etalon back surface, the second etalon back surface being separated from second etalon front surface, the second etalon being oriented relative to first beam and the second beam to reflect the first beam and second beam, and a first pair of mirrors oriented to cause the first beam and the second beam to interfere at a surface, the first pair of mirrors comprising a first mirror oriented to reflect the first beam and a second mirror oriented to reflect the second beam, the first etalon, second etalon, and first pair of mirrors, being selected and arranged to have the first beam and second beam have the same orientation at the surface.
A third aspect of the invention is a method of projecting an interference pattern, comprising (a) projecting a beam, (b) non-diffractively dividing the beam by reflecting and refracting the beam to produce a first beam and a second beam, and (c) intersecting the first beam and second beam to form an interference pattern at a surface, the dividing and intersecting steps combining to have the first beam and second beam reach the surface with the same orientation. Optionally, the first beam and the second beam produced during the dividing step propagate in substantially parallel directions, and at a plane normal to the directions, have traversed substantially the same optical path length. In one embodiment of the third aspect of the invention, the first beam and second beam produced by the dividing step are TE polarized.